A little oil left over from frying your breakfast eggs can easily be sopped up with paper towels or an upcycled cotton rag. But some recipes—hello, doughnuts!—leave larger quantities of oil behind.

As an author and blogger focused on greener living, I often hear from people striving to reduce their impact on the environment. Household waste, including used cooking oil, is a big piece of that puzzle. So I was naturally intrigued when I heard about so-called cooking-oil solidifiers, a product touted as a solution to the waste oil dilemma.

To study the problem and possible solutions, let’s return to that panful of oil.

Waste oil options

Depending upon what was cooked, you can reuse the oil a few times to make the most of it. But after several batches you’ll want to discard the old oil and start fresh for the best flavor.

Restaurants and commercial facilities have the ability to send their annual three billion gallons of waste cooking oil off to be transformed into biodiesel. Home cooks with a frying pan full of oil are often stymied at how to get rid of that greasy waste. Count yourself lucky if your municipality offers an oil recycling program, the best and easiest way to solve the problem. Not all of us have access to such a program.

Sending waste oil down the drain or flushing it down the toilet may make it disappear, but those oils can damage your home’s pipes and build up in municipal sewers. Oil and fats act as a glue, holding assorted sewer detritus together to create fatbergs, causing backups that require costly manual removal. 

Cooking oil and grease that make their way into the environment can also damage ecologically fragile shorelines and injure wild animals. In some U.S. communities, people have the option to compost small amounts of waste vegetable oil, but it’s a messy product to dispose of when dumping it in the trash is the only option. Sealing the oil into a disposable plastic container, as many home cooks do, adds a container to the waste stream that could perhaps have been recycled; it also isolates the oil, preventing it from breaking down.

Cleaner oil disposal

There are several brands of oil-solidifier products on the market, under different names. They’re purportedly made from eco-friendly ingredients that, when sprinkled into hot cooking oil, cause it to solidify for easy disposal. How does it work? Laura Lady, developer of the FryAway brand solidifier, explains that when the dry flakes are stirred into hot oil, the natural plant-based hydrogenated fats in the product melt, essentially emulsifying with the used cooking oil. 

It may seem counterintuitive to add more fat to resolve a greasy mess. But as the combined fats cool to about 100 degrees Fahrenheit, the mixture thickens into a texture that’s easier to dispose of than liquid oil. The cooled oil’s density falls somewhere between the liquid state of cooking oil and the dry, solid flakes of the additive. It may become a wobbly-but-solid disk or a thick, somewhat dry paste that’s easy to scrape out of the cooking pot. 

Small amounts of oil can take as little as 20 minutes to solidify; larger amounts can take several hours. Once cooled, the firm oil can be lifted or scraped, mess free, from the fryer or cooking pan and tossed directly into the trash, the green waste bin, or—in reasonable quantities—a compost bucket. Sending this to the landfill rather than sealing waste oils into a plastic container for disposal means that microbes can access the waste and break it down.

The solidified oil can also be retained for household use: It’s perfectly suitable for mundane tasks such as oiling hinges. Product developer Laura Lady says that transferring the mixture to a container with a candle wick makes an inexpensive DIY candle.

A less costly option

You may be convinced that you need this product in your life, but the expense and the plastic packaging associated with these products gave me pause. Enter stearic acid, available online and at many retail stores.

A long-chain fatty acid, stearic acid is made from both animal and vegetable fats; the waxy appearance of its thin flakes makes it look quite similar to the commercially available fat solidifiers. As it turns out, adding two or three tablespoons of stearic acid to a cup of hot oil works in a similar fashion—for half the price and often with less packaging.

Kris Bordessa is the author of the National Geographic book Attainable Sustainable: The Lost Art of Self-Reliant Living, and a blogger at attainable-sustainable.net.

A version of this story appears in the February 2023 issue of National Geographic magazine.

By Taba